1. Field of the Invention
The present invention relates to a continuous drying apparatus for a porous web, suitable for use in a pressure drying apparatus applied to the dryer part of a paper machine, a pressure drying apparatus for a porous web other than paper (e.g., a sheet drying apparatus), or the like.
2. Description of the Related Art
FIG. 4 is a schematic diagram showing a conventional continuous drying apparatus for porous web (citation from Japanese Patent Publication No. HEI 1-56198). In this apparatus, as shown in FIG. 4, both a porous web 3 (such as paper, a sheet or the like) to be dried and a drying band (e.g., a drying felt or wire) 4 for supporting this porous web 3 enter an air removing chamber 6 along with an auxiliary wire 5. After being subjected to an air removing process, they are passed through between two surface elements 1 and 8 having satisfactory heat conductivity and air non-permeability.
Here, the above-mentioned surface elements 1 and 8 interpose the porous web 3 therebetween over the entire width. The surface element 1 in contact with the porous web 3 is heated by a heating medium in a heating space 2. Furthermore, the surface element 8 in contact with the drying band 4 is cooled by a liquid flowing through a cooling space 11.
With this, the porous web 3 is heated through the surface element 1, whereby the moisture contained in the porous web 3 vaporizes and turns into steam. On the other hand, since the drying band 4 is cooled through the surface element 8, the steam vaporized from the porous web 3 condenses into water within the drying band 4. In this manner, the water (moisture) contained in the porous web 3 is gradually removed by external heating and cooling, so that the drying of the porous web 3 is continuously performed.
Also, after the drying band 4 is separated from the surface elements 1 and 8, it is separated from the porous web 3 and the condensed water within the drying band 4 is removed at a suction box 17.
Furthermore, the cooling space 11 is sealed through appropriate seals 16a and 16b with respect to a hood 13 supported by support beams 14 and to rolls 9 and 10. The cooling liquid flowing through this cooling space 11 is supplied from a liquid supply port 12 and exhausted from a liquid exhaust port 15.
However, in such a conventional continuous drying apparatus for porous web, the cooling liquid flowing through the cooling space 11 is sealed by the rolls 9 and 10, so there is a problem in that the cooling liquid will adhere to the surfaces of the rolls 9 and 10 and therefore the surface element 8 will slip on the rolls 9 and 10. Particularly, in the case of running at high speed, this slippage becomes significant, wear on the drying band 4 becomes noticeable, and furthermore, the meandering of the drying band 4 becomes significant, so that stable running is obstructed.
In addition, the space between the hood 13 and various members, which constitute the cooling space 11, is sealed and the support beam 14 is increased in size due to pressure-proof structure, so there is also a problem that substantial time and labor will be required in replacing the surface element 8 or the drying band 4. More specifically, since the surface element 8 and the drying band 4 have an endless structure, they must be slid made to slide and replaced in a direction perpendicular to the paper surface of FIG. 4.
Furthermore, in the continuous drying apparatus for porous web shown in FIG. 4, a closed space is formed upstream of the cooling space 11 serving as a drying section (more specifically, a range from the liquid supply port 12 to the liquid exhaust port 15). An air removing chamber 6 is provided in the closed space. With this, the air 7 in the closed chamber 6 is continuously exhausted with a suction pump, whereby an air removing process is performed. However, in order to increase the drying speed, the pressure within the closed space has to be reduced to about 1 Torr or less. For this reason, there is also a problem in that the exhausting speed of the suction pump will become too high.
The trial example of the required exhausting speed is shown as follows:
(1) Conditions PA1 (2) Calculation of exhausting speed
a. Drying Band: Width B.times.thickness t.times.void ratio .PHI.=6 m.times.0.003 m.times.0.3 PA2 b. Line Speed: u=1200 m/min PA2 c. Degree of Vacuum: P.sub.1 =1 Torr PA2 S=Bt.PHI.u.times.760/P=6.times.0.003.times.0.3.times.1200.times.760/1=4.92. times.10.sup.3 m.sup.3 /min=4.92.times.10.sup.6 liter/min in which S=exhausting speed(m.sup.3 /min or liter/min).
As specifications for the suction pump, an oil-sealed rotary vacuum pump or a mechanical booster pump is selected from the condition of the degree of vacuum. These characteristics are shown in FIGS. 5 and 6, respectively.
As shown in FIGS. 5 and 6, even the conditions at which the required exhausting speeds (liter/min) respectively become maximum (the condition (1) in both FIGS. 5 and 6) are around 1.times.10.sup.4 1/min at a degree of vacuum of 1 Torr (pressure P.sub.1). In other words, the above-mentioned calculation result (4.92.times.10.sup.6 liter/min) is 100 times these general specifications and is therefore far from realistic.
Furthermore, FIG. 7 shows the influence of air (noncondensable gases) on the condensation heat transfer rate of steam. As shown in FIG. 7, as the air content in steam becomes higher, the diffusion movement of steam is blocked. This results in a reduction in the condensation heat transfer rate. A range that can neglect such an influence of air is air content rate&lt;about 0.002 kg (air)/kg (steam). The range is also air content rate&lt;about 0.001 m.sup.3 (air)/m.sup.3 (steam) in terms of a volume ratio. In other word, partial air pressure is equivalent to 1 Torr or less with respect to the total pressure 1000 Torr of atmospheric pressure.